In general, semiconductor-based photonic devices, such as photovoltaic cells (PV cells) include a junction formed between p-type and n-type conductivity regions in a semiconductor body. These conductivity regions generate a voltage potential and/or a current across the junction when electron-hole pairs are created in the semiconductor body in response to photons impinging upon the photovoltaic cell. When a load is connected between the p-type and n-type conductivity regions, an electric current will flow, thus producing power. PV cells therefore provide power from a renewable source, which is an attractive alternative to non-renewable energy sources.
The power conversion efficiency of a PV cell consisting of a single pn junction, referred to as a single junction solar cell, depends on the voltage that can be generated by the pn junction and the ability of the semiconductor(s) comprising the pn junction to absorb a significant portion of the solar spectrum. A semiconductor with a relatively larger bandgap energy can generate a large voltage, but photons having energies less than the bandgap energy are not absorbed by the semiconductor, and the generated current is relatively low. Likewise, a semiconductor with a relatively smaller bandgap energy can absorb a large portion of the solar spectrum and generate a high current, but photon energy in excess of the bandgap energy is lost to thermal energy, and the output voltage of the single junction solar cell is low.
Combining multiple pn junctions within a PV cell to form what is referred to as a multijunction solar cell can reduce the tradeoffs between voltage and current generation in a single junction solar cell and increase the energy conversion efficiency of the device. In a multijunction solar cell, the pn junctions are typically arranged in a vertically stacked configuration. The pn junctions may be stacked either through physical bonding or mechanical stacking of individual pn junctions that have been grown on separate substrates, or through monolithic integration of the pn junctions on one substrate. Each pn junction is designed to absorb a portion of the photons in the solar spectrum while passing photons with energy less than the bandgap energy to the pn junction(s) situated below. The use of multiple pn junctions in a multijunction solar cell therefore reduces thermalization losses. Multijunction solar cells therefore typically have efficiencies that are higher than single junction solar cells.
The manner in which the pn junctions that make up the multijunction solar cell are electrically connected together determines the voltage and current output of the device. A single pn junction solar cell can be partitioned by subdividing it into multiple units of individual PV cells arranged laterally, referred to as sub-cells. If the sub-cells are electrically connected in series, their voltages add, and the lowest current-producing sub-cell determines the overall current output of the device. If the sub-cells are electrically connected in parallel, the currents of each of the sub-cells add, and the voltage will be limited to an intermediate voltage between the highest and lowest values produced by any of the sub-cells. Multijunction solar cells may utilize one or both of these electrical connection configurations.
Conventional multijunction solar cells are configured such that vertically-stacked pn junctions are connected in series. These devices are typically referred to as current-matched multijunction solar cells because the individual pn junctions are usually designed to have the same current output. In a monolithically integrated device, the electrical connections between adjacent pn junctions are made with a tunnel junction. The tunnel junction is an ultrathin pn junction composed of heavily doped high bandgap energy p-type conductivity and n-type conductivity regions (heavily doped being defined herein as dopant concentrations of greater than about 1018 cm−3). From a manufacturing standpoint, tunnel junctions present a convenient way to “hard-wire” connections between vertically-stacked pn junctions, but achieving adequately high doping concentrations is difficult or impossible in many semiconductor materials. This is particularly true for many thin film semiconductors that are used in polycrystalline single junction and multijunction solar cells.
Voltage-matched multijunction solar cell designs can circumvent some of the limitations of current-matched designs. Variations in the solar spectrum throughout the day can have a large impact on the current that is output by individual pn junctions, but spectral variations will have a much smaller impact on the voltage output. Therefore, the performance of voltage-matched multijunction solar cells suffers less from diurnal spectral variations than current-matched multijunction solar cells. Moreover, tunnel junctions are not required between vertically-stacked pn junctions in a voltage matched design. This is advantageous when designing multijunction solar cells utilizing semiconductors in which it is difficult to achieve heavily doped ultrathin layers. The primary disadvantage of voltage-matched multijunction solar cells, however, is the need for more complex intra-cell electrical connections. Sub-cells formed within the same pn-junctions are typically connected in series, forming a sub-cell string, and the strings from the different vertically stacked pn-junctions are then connected in parallel. The number of sub-cells in each of the strings need not necessarily be the same and are chosen so that the voltage outputs of all of the strings match. Isolating and connecting sub-cells in pn junctions that are buried within monolithically-integrated multijunction solar cell stacks is challenging because it usually requires physical removal of material from overlying pn junctions in order to access buried pn junctions. In these cases, fabrication requires multiple etch and growth steps, which can add to the cost of the solar cell module. It also necessarily constrains the geometry and layout of the sub-cells in each pn junction layer.
Polycrystalline or amorphous thin film solar cells, including cadmium telluride (CdTe), copper indium gallium selenide (sulfide) (CIGS), copper zinc tin sulfide (CZTS), polycrystalline or microcrystalline silicon (Si) and amorphous Si (a-Si) architectures, have many advantages over crystalline Si or III-V solar cell technologies. For example, polycrystalline or amorphous thin film designs feature relatively lower overall material usage as compared to crystalline silicon cells, and the ability to fabricate cells on large area glass substrates with atmospheric deposition techniques reduces manufacturing and module costs. One trade-off is that single junction polycrystalline and/or amorphous designs have comparatively lower conversion efficiencies than their crystalline Si or III-V counterparts.
Multijunction solar cell designs incorporating thin film materials would capitalize on inexpensive processing costs while providing an avenue to increased efficiencies over single junction polycrystalline or amorphous thin film solar cells. Thin film solar cell designs featuring monolithically integrated structures in which the individual vertically stacked pn junctions are connected in series require that the output of these individual pn junctions be substantially current matched. As noted above, this requirement necessitates that tunnel junctions are formed between the monolithically grown vertically stacked pn junctions to facilitate current flow. However, difficulty in achieving heavily doped ultrathin layers in thin film materials impedes the formation of low resistance tunnel junctions making the production of thin film current matched solar cells problematic. Furthermore, the complexity of processing steps required by known prior-art voltage matched approaches (whether monolithic or mechanically stacked) makes known voltage matched solar cell technologies costly and thus unattractive.
The embodiments disclosed herein are intended to overcome one or more of the limitations described above. The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.